This invention relates to a calibrating method or method of determining an operating pressure required to achieve initial engagement of electro-hydraulically operated transmission brake or clutch elements in a powershift transmission.
Some manufacturers have used versions of electrohydraulic transmission controls with some success. Some such control systems have utilized proportionally controlled valves, but most such systems have used mostly simple on-off valves, with just one or two transmission control clutches controlled by proportional valves. For example, a powershift transmission sold by Ford New Holland, Ltd. has two modulating valves which control three different transmission control clutches. These valves are in turn controlled by an electronic controller. Such a proportional control allows a clutch element to be modulated during engagement and release of that element, and the controller provides the ability to vary the modulation for a particular element for each unique shift. Another system which includes on-off valves and at least one proportional control valve is described in U.S. Pat. No. 4,855,913, issued 8 Aug. 1989 to Brekkestran et al.
In such systems with proportional control valves it is possible and desirable to accurately control the torque capacities of the clutches during engagement. While the electrical command supplied to the control valve may be very precise, manufacturing tolerances in the valves and transmission cause large variations on an actual vehicle. If it is known what electrical command corresponds to the initial clutch engagement pressure which causes a clutch to just begin carrying torque, then this command could be used to modify the shift command for that clutch during shifting to provide optimized control.
For example, the Brekkestran reference discloses that the key parameters in the control system include the initial clutch engagement pressure (represented by DC-MAX) and the fast-fill clutch delay (represented by T1). The Brekkestran reference further states that DC-MAX and T1 must be determined by laboratory or field tests. However, the Brekkestran reference does not disclose any method for determining these values.
A calibrating method or a method of determining the pressure necessary to achieve clutch engagement in a microprocessor-based transmission control system is described in U.S. Pat. No. 4,989,471, issued on 5 Feb. 1991 to Bulgrien. The Bulgrien method includes braking the transmission output shaft, then gradually increasing the clutch pressure and saving a value corresponding to the clutch pressure at which the engine speed begins to decrease. However, this method requires use of the vehicle brakes to prevent rotation of the transmission output shaft. Such a procedure could be dangerous if the vehicle brakes are not applied or if the brakes fail, because then undesired vehicle motion could result during calibration. The Bulgrien patent also illustrates an alternate method of calibrating a clutch by sensing when the clutch transmits sufficient torque to move the vehicle. This alternate method requires that the vehicle be placed in a position where vehicle motion is not a safety concern, and the results of such a method will vary depending upon the terrain on which the vehicle is placed.
U.S. Pat. No. 5,082,097, issued on 21 Jan. 1992 to Goeckner et al. relates to a transmission controller for a transmission which includes a solenoid valve operated clutch and a solenoid valve for operating the clutch. The Goeckner et al. system also discloses a calibrating system or a system for determining a current signal corresponding to the point at which the clutch begins to transmit torque. This system includes a vehicle monitor for producing a threshold signal when the clutch begins to carry a predetermined amount of torque and a memory for storing a calibration value corresponding to the value of the current at which the clutch carries the predetermined amount of torque. However, this system requires a controller which generates a controlled current signal and a current monitoring circuit which generates a signal which corresponds to the current flow to the solenoid valve. Furthermore, the threshold signal in this system is either vehicle movement or engine droop, and the calibration procedure does not appear to involve disconnecting an output shaft of the transmission from a vehicle drive shaft. As a result, the calibration procedure described in Goeckner et al. would appear to be similar to the methods described in the Bulgrien patent, and would appear to have to involve allowing vehicle movement during calibration, or applying the vehicle brakes during calibration.
Another calibration method is described in U.S. Pat. No. 5,224,577, issued 7 June 1993 to Falck et al. and assigned to the assignee of the present application. With this method, the transmission output shaft is disconnected from the drive wheels and the engine is run at a predetermined speed. Depending upon which transmission control element is to be calibrated, certain ones of the plurality of the control clutches, other than the control clutch being calibrated, are engaged in order to prevent rotation of a part of the control clutch being calibrated. Then the duty cycle of a pulse-width-modulated voltage signal applied to the proportional control valve is gradually modified to increase the pressure applied to the control clutch being calibrated while the engine speed is monitored. When the monitored engine speed droops by a predetermined amount, a value corresponding to the pressure applied to the proportional control valve is stored as the clutch calibration pressure value. In order to determine the fill volume of an element, the element is fully pressurized while the engine speed is monitored. The time required for this for this pressurization to cause a decrease in engine speed will represent the volume of fluid required to fill the element so that it begins to carry torque. While this method does not require that the brakes be applied to prevent vehicle motion, it does require an engine speed sensor and it requires that the engine be run at a certain speed.